1. Field of the Invention
The present invention relates to the stabilization of halogenated polymers, notably PVC, PVDC and mixtures and copolymers thereof, and to the halogenated polymer compositions thus stabilized.
2. Description of the Prior Art more particularly, of chlorinated polymers, using zinc or cadmium salts in combination with salts of metals of Group IIa of the Periodic Table of the elements has long been known to this art. However, this stabilization has been found to be insufficient in the case of applications requiring good resistance to yellowing.
To improve such yellowing resistance, it has been proposed to the art to add secondary organic stabilizers to the aforesaid primary stabilizers.
.beta.-Diketones and .beta.-ketoaldehydes are particularly effective among the former additives. They permit the formulation of compositions which are colorless and whose color exhibits practically no change during their processing. This is the reason that their use is very widespread in applications such as the production of polyvinyl chloride bottles.
However, it is sometimes necessary to recycle chlorinated polymers, and this increases the total duration of the heat treatment to which they are subjected.
Moreover, during the processing of chlorinated polymer compounds, whether by accident or as a result of poor programming of the heating cycle, it is not uncommon that the polymer is subjected to a higher temperature or to a longer period of heat treatment than normally.
Thus, substantial need exists in the chlorinated polymer processing art for an extended working range, namely, a heating time during which the initial color undergoes very little or no change, which is longer than presently available, while maintaining the benefit of the colorless and transparent nature imparted by the .beta.-diketones.
Various solutions have been proposed to the art to accomplish these results.
Thus, published French Patent Application FR-A-2,356,674 describes the addition of a polyol which makes it possible, in particular, to lengthen the blackening resistance, sometimes designated "long-term resistance", and, to a lesser extent, to lengthen the working range of the polymer
However, the addition of a polyol does not enable the working range to be sufficiently improved for all compositions, and especially in compositions which, for example, contain less than 0.5% of zinc compounds.
It is also very common to introduce epoxidized oils into chlorinated polymer compositions, such as epoxidized soya oil, which also promotes a lengthening of the working range of the polymer when heated.
However, a disadvantage of these epoxy compounds, especially of epoxidized soya oil, which is very widely used, is that they lower the softening point of the polymer, in particular when large amounts are employed. The amount of these adjuvants must therefore be reduced, while at the same time maintaining the beneficial effect which they provide.